Commonly, devices for subcutaneous medical applications require a motor to couple a moving part in a fluid environment. Fluid environments can present hazardous conditions for certain motors. One example is in the use of ultrasound for imaging, therapy or other medical uses. In such use, ultrasound energy or waves are transmitted through a medium and can reflect, scatter or otherwise attenuate when they reach a surface or border having a significant difference in acoustic impedance. For example, in ultrasound imaging of the human body, ultrasound waves may be applied externally (e.g. by placing a transducer on the skin) or internally (e.g. by placing a transducer within a vessel or organ), and travel through the body's internal fluids, which is a large proportion of water. When the waves strike a bone, organ or other body portion that provides an acoustic interface—i.e., a border of two significantly different acoustic impedances—then the waves are reflected or otherwise attenuated. A transducer (which may be the same transducer that supplied the ultrasound waves or another) receives the reflected or attenuated waves, and an image of a portion of the body can be generated.
A gel is placed between the skin and transducer to reduce reflection or other attenuation between the transducer and the skin. When a transducer is placed within the body, commonly it is inside a protective envelope, such as a tube, catheter or similar housing or enclosure. The material of such an envelope may be selected for its similarity in acoustic impedance to that of bodily fluids, so that there is little or no attenuation as ultrasound waves travel from that material to the fluids or tissues of the body. The inner pocket or volume of the enclosure within which the transducer is placed needs a coupling medium having an acoustic impedance similar to that of the envelope material and the body's fluids, to allow maximum transmission of the ultrasound signal. Without such a medium, e.g. if the inside of the body simply includes air or another gas, significant reflection or other attenuation will occur when the ultrasound energy from the transducer hits the boundary where the gas meets the material of the envelope. Suitable coupling media include biocompatible fluids such as saline, oils such as mineral oil, alcohols, and other fluids.
In ultrasound devices in which the transducer can turn, pivot or otherwise move, a seal between or around mechanisms to move the transducer may be necessary to limit or prevent the coupling medium from corroding, fouling or otherwise interfering with performance of such mechanisms. For example, in mechanisms using a motor that operates a turning or otherwise mobile shaft, with the shaft connected to the transducer or its seat or other holder, a seal may be needed between a chamber holding the transducer and coupling medium and the motor. In another example, a motor is combined with a tool to drill a bore through a clot or plaque in a vein or artery. Corrosive and/or electrolytic coupling media may be incompatible with electrical connections or other parts of a motor, drive shaft or other mechanism. Piezoelectric motors generally need dry conditions to operate, as they require a high friction contact area between a stator and a clutch. If fluid (whether generally corrosive or not) touches that contact area or interface, the friction will be substantially reduced, thereby also reducing the torque output of the motor.
Accordingly, to prevent fluid from contacting parts of such devices, such as motors, a seal should be included between the motor and the fluid environment, to prevent the fluid from gaining access to the motor. Examples of such a structure are disclosed below.